The present invention relates to a temperature-sensitive switch, in particular for electrical parts to be protected against excess temperature and/or excess current, such as e.g. electrical motors and transformers.
A prior art temperature-sensitive switch is known from document DE-A 43 36 564. The known temperature-sensitive switch comprises a bimetallic switching device opening or closing its contacts in response to an excess temperature, a casing having a pot-like lower part and a cover part, for containing said switching device, a first heating resistor connected in circuit with said switching device such that it locks said switching device in a self-holding manner when said switching device is actuated, and a second heating resistor connected in circuit with said switching device and producing heat in response to a current flow therethrough, such that said switching device is actuated by an excess current flowing through said second heating resistor.
The first heating resistor is connected in parallel and the second heating resistor is connected in series to the switching device.
The known temperature-sensitive switch consists of a ceramic base plate provided with conductive and insulating coatings on which an encapsulated bimetallic switching device is arranged, alongside which there is a posistor module (PTC resistor) connected in parallel to the bimetallic switching device which functions as the first heating resistor. The ceramic base plate also bears a thick film resistor leading beneath the bimetallic switching device which is connected in series to this circuit.
The object of the known temperature-sensitive switch is to interrupt the current flow through the electrical part if the temperature of the part or the current flowing through the part become too high. For this purpose the known temperature-sensitive switch is connected in series to the part so that the current flowing through the part also flows through the temperature-sensitive switch, whereby the bimetallic switching device remains closed at temperatures below the release temperature and/or at currents below the release current.
The operating current of the part flows through the second heating resistor of a few ohms, which is connected in series, and through the closed contacts of the bimetallic switching device, which bridges the first heating resistor. If the temperature of the part now exceeds a given threshold value the bimetallic switching device, which has thermal contact with the part, suddenly opens its contacts inasmuch as a bimetallic snap switch inside the bimetallic switching device is triggered. The current now flows through the series connected heating resistor and through the second heating resistor which displays such a high resistance that the current is much lower than the original operating current so that the part is quasi switched off. On account of the posistor characteristics of the second heating resistor the current drops when this heating resistor heats up. As a result of the heat radiated and/or conducted by this heating resistor the bimetallic snap switch continues to be heated so that it is automatically locked in its position with the contacts open. This prevents an automatic reclosure if the part which has been switched off on account of an excess temperature cools down, which could lead to so-called "contact flutter" with periodic reconnections and disconnections and which is generally undesired.
However, if the current flowing through the part, and thus also the bimetallic switching device, and not the temperature reaches a given threshold value the series connected heating resistor heats up to a point where the switching device finally reaches its release temperature and opens. The switch is hereby locked in the same manner as described above.
Although the known temperature-sensitive switch meets all functional requirements its disadvantage is its relatively bulky and large size, due particularly to the ceramic base plate. For reasons of installation and thermal capacity such temperature-sensitive switches are normally of a very small design, for example with a diameter of 10 mm and a height of 5 mm, which places extreme demands on the manufacturing accuracy and is also the reason for the necessity of a simple and at the same time functionally reliable construction.
Such a miniature temperature-sensitive switch is known from EP-A 0 342 441 and DE-A 37 10 672. This temperature-sensitive switch is of a locking embodiment, though it displays no excess current sensitivity. In other words, the known temperature-sensitive switch comprises a heating resistor connected in parallel to the bimetallic switching device whose effect is similar to that described above in connection with the first heating resistor. There is no series connected second heating resistor.
In order to keep the overall size of the known temperature-sensitive switch small the high-resistance parallel resistor is integrated in the casing of the bimetallic switching device. This casing comprises a pot-shaped bottom part and corresponding cover made of either insulating material or an electrically conductive high-resistivity material.
The casing contains a bimetallic snap switch and spring washer bearing a movable contact which is assigned a fixed contact borne by the cover. The spring washer presses the moveable contact against the fixed contact and simultaneously conducts the current flowing through the contact to the bottom part, to which a first external contact is fixed. The second external contact of the known temperature-sensitive switch is arranged on the cover and makes electrically conductive contact with the fixed contact of the bimetallic switching device through the cover. The bimetallic snap switch acts on the spring washer and if a given release temperature is exceeded it suddenly snaps, thus raising the moveable contact from the fixed contact so that the flow of current through the bimetallic switching device is interrupted.
The current now flows through the parallel connected heating resistor and thus leads to the aforementioned locking. This heating resistor can consist either of the high-resistivity material of the cover or can be printed on the cover if this is made of insulating material.
The disadvantage of this known temperature-sensitive switch is that if provides no protection against excess current. A further disadvantage is that the design variant where the cover is made of electrically conductive high-resistivity material requires an insulating envelope between the cover and bottom part to ensure a defined current path and thus a defined resistance. However, if the heating resistor consists of a printed strip resistance, the disadvantage here is that this strip resistance must be helical and/or in curves so as to achieve the desired resistance value and current path. The disadvantages in both cases relate to the high manufacturing costs.
A similar miniature embodiment for a temperature-sensitive switch is also known from DE-A 41 42 716, without a locking function through a parallel connected heating resistor, though with a series connected heating resistor integrated in a very small space which monitors the current. The protective resistor is arranged as an etched or punched part or as a film printed with a resistor in the immediate vicinity of and in thermal and electrical contact with the spring washer of the bimetallic switching device in such a way that it lies in the base of the bottom part of the casing.
Apart from the complicated assembly of the known temperature-sensitive switch a further disadvantage is that the etched or punched parts used as heating resistors are relatively inaccurate with respect to the resistance values and can only be manufactured for a small resistance range. An additional insulating part between the casing bottom and the heating resistor and in most cases an additional externally mounted high-resistance resistor in series with the aforementioned protective resistor are needed for reasons of resistance adjustment, which increases the overall manufacturing costs and overall size.
Bimetallic safety switches are generally known in a pot shape and only display one of the two protective functions mentioned at the beginning with respect to temperature and current.
DE-A 36 32 256, for example, describes a temperature-sensitive switch with locking function which only reacts to excess current, whereby the heating resistor consists of a freely clamped resistance wire spiral close to the bimetallic element. The disadvantages here are the high spatial requirements, possible fluctuations in the orientation to the bimetallic element and corresponding fluctuations in thermal transfer as well as contact problems at the connections of the resistance wire spiral.
From DE-C 34 01 968 it is known that the electrical contact of the bimetallic element can even be made of a high-resistance material so that this also heats up at excess current, leading to a steeper characteristic curve for the triggering of the temperature-sensitive switch depending on the amount of excess current. The transfer of heat to the bimetallic element is hereby better and safer than in the aforementioned solution, though the spatial requirements, particularly in a radial direction, are so high that a pot-shaped embodiment is no longer possible. Moreover, the resistance elements have a complicated shape which is difficult to manufacture whereby a direct passage of current through the bimetallic element itself leads to a more inexact switching than with a bimetallic element which is only heated by a special protective resistor.